Variable pitch coil filaments providing uniform temperature throughout

ABSTRACT

The catalytic metal filaments of gas detectors and the like are in coil form with progressively increasing pitch toward their middle portions whereby excessively high temperatures at such middle portions are avoided. The substantially uniform operating temperatures throughout most of the lengths of such coil filaments result in dependably good performance and greatly lengthened useful lives of the filaments.

United States Patent Archer [45] May 23,1972

[54) VARIABLE PITCH COIL FILAMENTS [56] References cm PROVIDING UNIFORM TEMPERATURE THROUGHOUT UNITED STATES PATENTS 3,476,517 11/1969 Smith ..73/27 X Lee A. Archer, Wheaton,1ll.

Erdco Engineering Corporatlon, Addison, Ill.

lnventor:

Assignee:

Filed: June 15, 1970 Appl. No.: 46,012

Related U.S. Application Data Continuation-in-part of Ser. No. 696,835, Jan. 10, 1968, Pat. No. 3,522,010.

U.S. Cl ..3l3/344, 313/271, 313/311 Field ofSearch ..3l3/27l279, 311, 313/344; 73/27 3,538,374 11/1970 Kane ....3l3/344X 3,431,448 3/1969 English ..3l3/344X Primary Examiner-David Schonberg Assistant Examiner-Toby H. Kusmer Attorney-Darbo, Robertson & Vandenburgh ABS lRACT The catalytic metal filaments of gas detectors and the like are in coil form with progressively increasing pitch toward their middle portions whereby excessively high temperatures at such middle portions are avoided. The substantially uniform operating temperatures throughout most of the lengths of such coil filaments result in dependably good performance and greatly lengthened useful lives of the filaments.

8 Claims, 3 Drawing Figures PATENTEDMAY 23 m2 /72 50] l 6 6 Q. arch 6] M. m MW VARIABLE PITCH COIL FILAMENTS PROVIDING UNIFORM TEMPERATURE THROUGHOUT RELATED APPLICATION This application is a continuation-in-part of application Ser. No. 696,835, filed Jan. 10, 1968 now U.S. Pat. No. 3,522,010.

BACKGROUND OF THE INVENTION The invention herein described relates to improvements in the catalytic filaments (e.g., platinum wire) used in combustible gas detectors and the like.

When an electrically heated filament, such as the detector or reference filament of combustible gas detecting apparatus, is exposed to gases or vapors of changing condition or composition, a change in the temperature of the filament will result. This change is largely due to chemical reaction such as catalytic combustion at the hot catalytically active filament surface. The change in temperature of the filament is normally accompanied by a change in the resistance of the filament and therefore the current carried thereby. This change can conveniently be measured by means of a Wheatstone bridge.

Gas detectors utilizing these basic principles to a greater or a lesser extent are in common use. The dependability of these instruments is determined largely by the physical properties and longevity of the sensing filaments. These elements are very expensive and must be replaced from time to time since the material of which they are made slowly evaporates or sublimes at the elevated temperatures at which they are maintained during operation of the apparatus.

SUMMARY OF THE INVENTION The object of this invention is to provide an improved design for the filaments of combustible gas detectors and the like which will have greatly extended service lives.

The filaments generally employed in apparatus of this type comprise helical coils of platinum wire or other suitable wire form materials. Excessive and localized loss of the metal due to excessively high temperatures of the middle portion of the coil may be avoided by progressively increasing the pitch of the coil windings from the ends toward the center portion to thereby provide a more uniform temperature throughout the length of the coil. By this expedient, the useful service life of the very expensive filaments may be greatly extended since loss of metal is minimized and uniform over the length of the coil no part of which is at a temperature greatly in excess of the functionally necessary temperature.

DESCRIPTION OF THE DRAWING In the accompanying drawing,

FIG. 1 is a side view, partly in section, of a diffusion type sampling head for a combustible gas detector embodying the filament of the invention;

FIG. 2 is a side view of a typical coil filament of the prior art with a graph representing the operating temperature throughout its length, and

FIG. 3 is a side view of a coil filament in accordance with the invention with a graph representing the operating temperature throughout its length.

DESCRIPTION OF PREFERRED EMBODIMENTS The improved form of coil filament is described as used in a typical useful application, a sampling head of a combustible gas detector.

The supporting base structure of the sampling head illustrated in FIG. 1 is more or less conventional and will not be described in detail. A porous diffusion cap 1 of sintered metal or other inert material, secured to the base portion of the head by means of ferrule 2, provides an enclosure for the sensing filament subassembly, designated, generally, by the numeral 3. This diffusion cap provides a relatively quiescent space within which the filaments operate, permitting the air and other gases surrounding the head to diffuse into and out from the interior of this space for sampling and detection purposes.

The air within the sampling head is constantly in motion, flowing over the filaments to enable the latter to signal the combustible gas content, if any, of the air. Convection currents are created and maintained by the hot filaments and the filament subassembly 3 is so constructed that the currents are brought to bear upon the respective filaments with velocity and direction calculated to produce the desired signal characteristics from the instrument. As will be seen as the description proceeds, provision is made for the continuous scouring of the detector filament by raw sample air while maintaining a less active but changing air environment at the reference filament. In certain positions of use of the sampling head, advantage is taken of the stream of air leaving the detector filament and containing the products of combustion of the burned gases to shield the reference filament from raw air.

The sampling head with filament subassembly 3 shown in FIG..1 is intended for use in the position shown in the drawing, that is, directed vertically upwardly. An open-topped chimney 4, preferably metallic, is mounted upon a base 5 of insulating material which also serves as a supporting header for metal pins 6, 7 and 8, which, in turn, support detector filament 9 and reference filament 10 and make electrical connections therewith. Electrical connection with the pin conductors is provided by prongs 6a, 7a, and 8a. Four small openings 11 are provided in the bottom end of the chimney 4.

In the operation of the device, electrical currents of sufficient magnitude to heat the filaments to a temperature at which combustion of combustible gases passing along the surfaces of the filaments will be catalytically oxidized are continuously supplied to the filaments. Ambient gases are heated by these hot filaments and by any combustion that takes place at the filament surfaces and convection currents are consequently induced within the chimney 4, gases being drawn into the chimney through openings 11 and leaving at the open top. This continuous movement of gases takes place regardless of the presence or absence of combustible component gases or vapors. This convection action also induces passage of gases through the diffusion cap 1 to provide a continuing supply of sample gases from the space surrounding the sampling head.

In accordance with the intended operation of the device, combustible gases present in the sample flowing into the chimney through openings 11 are caused to burn as they pass by the detector filament 9 due to the catalytic effect of the platinum or other filament metal. This results in increasing the temperature, and, therefore, the resistance, of the detector filament. The air containing the products of combustion from detector filament 9 rise with the-convection currents within the chimney, as is indicated by arrows 12, providing a substantially inert stream core clinging to the hot wires and flowing over the reference filament 10 so that there is little or no combustion effect on this filament. In this way, the desired unbalance, indicating the presence of combustible gases in the gas sample, is achieved for signaling, as desired, by the apparatus.

To an appreciable extent, metallic chimney 4 tends to equalize the temperature of the gases ambient to the two filaments. However, to insure that detector filament 9 and reference filament 10 will have equal resistance values when the gas stream passing through the sampling head contains no combustible components, the detector filament may be made slightly longer than the reference filament to compensate for the normally somewhat higher temperature of the reference filament since it is influenced by the gases heated by the detector filament. Once calibrated, the response of both filaments to atmospheric and line voltage variations will be substantially identical and, consequently, there will be no zero drift.

The dependability and general performance of sampling heads made and operating as described have been found to be excellent when operated in upwardly directed vertical position, as shown, or within approximately 15 of vertical. The continuous scouring of the detector filament by the sample air convection currents assure vigorous and rapidly responsive signals from the apparatus.

The improved form of filament is illustrated schematically in the gas detector of FIG. 1. It is illustrated separately in FIG. 3 and, for comparison purposes, the heretofore standard form of coil filament is illustrated in FIG. 2.

When, as is illustrated in FIG. 2, the convolutions of the spiral wire filament are equally spaced throughout its length, i.e., from terminal to terminal the temperature of the middle portion is very much greater than that of the end portions which tend to lose heat continuously to connecting materials.

The pro forma graph of FIG. 2 indicates the temperature variations along the length of this form of coil filament. The minimum effective operating temperature of the filament is indicated by the broken horizontal line. It will be noted that in order to render most of the length of the filament functional, the temperature of the middle portion is unnecessarily great, resulting in excessive evaporation or sublimation of the filament with consequent shortened useful life. With the greater spacing of the convolutions in the middle portion, as is illustrated in FIG. 3, the temperature of the filament is substantially uniform from end to end of the coil and only slightly in excess of the minimum operating temperature indicated by the broken line. The useful life of the improved form of coil filament is approximately twice that of the conventional form and, in addition, a greater part of the length, and therefore functional surface, is operative in carrying out the intended function of the filament.

It will be understood that the improved form of coil filament is not limited to platinum wire filaments, but other metals and fabricated wire form elements may be employed. Palladium, nickel and other metals of Group VIII of the periodic table may be used, as may be platinum clad tungsten wire with or without a porous coating of palladium. For some applications,

it is desirable to provide a coating of a suitable oxidation catalyst, such as platinum oxide. For use in a combustible gas detector, such as that above described, the platinum wire forming the coil filaments may be coated with an inert material, such as aluminum oxide and the detector filament additionally coated with a catalyst.

I claim:

1. A catalytic metal wire filament for use at high maintained temperatures to catalyze chemical reactions comprising a helical coil of catalytic metal wire whereof the pitch of the turns forming the middle portion with respect to the terminal ends thereof is substantially greater than the pitch of the turns forming the end portions, whereby the temperature of said filament is substantially uniform throughout the length of said filament when used at said high maintained temperatures.

2. A filament in accordance with claim 1 wherein the catalytic metal is platinum.

3. A filament in accordance with claim 2 wherein the platinum wire is coated with an oxidation catalyst.

4. A filament in accordance with claim 2 wherein the platinum wire is coated with palladium.

5. A filament in accordance with claim 1 wherein the catalytic metal is palladium.

6. A filament in accordance with claim 1 wherein the wire comprises a tungsten core covered with platinum.

7. A filament in accordance with claim 6 wherein the platinum is coated with palladium.

8. A filament in accordance with claim 1 wherein the pitch of the coil turns increases progressively from the end portions of the coil toward the middle portion thereof. 

1. A catalytic metal wire filament for use at high maintained temperatures to catalyze chemical reactions comprising a helical coil of catalytic metal wire whereof the pitch of the turns forming the middle portion with respect to the terminal ends thereof is substantially greater than the pitch of the turns forming the end portions, whereby the temperature of said filament is substantially uniform throughout the length of said filament when used at said high maintained temperatures.
 2. A filament in accordance with claim 1 wherein the catalytic metal is platinum.
 3. A filament in accordance with claim 2 wherein the platinum wire is coated with an oxidation catalyst.
 4. A filament in accordance with claim 2 wherEin the platinum wire is coated with palladium.
 5. A filament in accordance with claim 1 wherein the catalytic metal is palladium.
 6. A filament in accordance with claim 1 wherein the wire comprises a tungsten core covered with platinum.
 7. A filament in accordance with claim 6 wherein the platinum is coated with palladium.
 8. A filament in accordance with claim 1 wherein the pitch of the coil turns increases progressively from the end portions of the coil toward the middle portion thereof. 